Mass analysis of ions selected from a group of ions can be achieved by extracting the desired ions from an ion beam. In particular, it is known to selectively extract ions from an ion beam by delivering ions to a linear ion storage device and trapping some portion of the ions in an ion trap. Trapping of the ions permits cooling of the ions and an ion cloud is formed in the ion trap. Ions are then extracted from the ion trap and subsequently pass into a mass analyser. WO2008/071923 describes such an arrangement in which a linear ion trap (LIT) comprising a plurality of segments is configured to filter, trap and extract ions. Each segment of the LIT has four rods of hyperbolic profile, arranged to provide a quadrupole field in the space between the rods. A schematic of this instrument 2 is shown in FIG. 1.
In one arrangement ions are introduced into the LIT in the form of a continuous beam and may be transported directly to and trapped within the extraction segment 6. The ions are then cooled in the presence of a buffer gas, before extraction in an orthogonal direction, via ion focusing elements 7 towards a Time of Flight (ToF) analyser 8 comprising detector 9. The application of a digital waveform for the radial trapping of the ions in the LIT assists in the control of the extraction of the ion cloud such that the extracted ions have the correct properties for acceptance by the ToF analyser 8. As is discussed in more detail in WO2008/071923, the LIT segments that precede the ion trap/extraction trap can be configured to provide a mass filter 10 and an isolation trap 12. Ions not extracted can be detected by detector 13.
In this approach to preparing ions for extraction the ions, or “cloud” of ions, are trapped within a fixed region of space. Indeed, WO2008/071923 describes ion trapping by a combination of high frequency signal with periodic time dependence, i.e. an applied RF signal to provide the radial confinement and a static field to provide the axial confinement. In this arrangement the ion cloud has an approximately cylindrical shape. The length of the cylindrical ion cloud is determined by the length of the segment in which the ions are trapped and the voltages applied to adjacent segments. An experimentally measured image of the ion cloud, after its extraction in an orthogonal direction, is shown in FIG. 2. It can be understood that the length of the extracted cloud determines the number of ions that can be trapped at one time.
The present inventors have noted that the total number of ions that can be stored is much greater than the number that can be stored without the onset of the effects which adversely influence the performance of the analyser. The main adverse effect is “space charge” interaction, which results in mass shift or loss of mass spectral resolving power. The space charge interaction is due to columbic repulsion between ions. Such adverse effects may occur not only within the ion trap, but also during ion flight towards and within the ToF analyser.
An alternative to the cylindrical ion cloud approach of WO2008/071923 is a Paul trap (3D ion trap) wherein the ion cloud is compressed approximately towards a single point. In such an arrangement, the ion density is correspondingly higher than the ion cloud formed in the LIT of WO2008/071923, which means that the ion trapping capacity is correspondingly lower, typically by 20 to 100 times.
Further methods to trap ions in the target segment of a LIT are described in F. Herfurth, Nucl. Instrum. Meth. A469 (2001) 254-275. This paper describes “a linear radiofrequency ion trap for accumulation, bunching, and emittance improvement of radioactive ion beams”. In this case the ions are initially collected over an axially extended region, spanning a plurality of segments of an LIT, before ions are finally collected into the target segment. This method allows for reduction in the buffer gas pressure without compromising the trapping efficiency, but collection and cooling times are relatively slow.
A still further method of trapping ions prior to ejection for mass analysis, referred to as “dynamic trapping”, was first introduced in EP1051734A1 (Shimadzu Research Laboratory) and is also described within patent application U.S. Pat. No. 6,670,606 (Perspective Biosystem, Inc.). In this method the ion traps are used for ion storage prior to ejection for ToF analysis. Such methods allow for trapping without the requirement for a buffer gas. Thus, in this method of ‘dynamic trapping’ ions are pulsed into the target trapping region from an external storage region, and the trapping is provided by the timed introduction of either RF or DC trapping fields. There are two drawbacks with this method, the first is that the trapping efficiency is strongly mass dependent so that the mass range may be limited. The second drawback is that a significant amount of energy is introduced into the ions, and at the low operating pressure a significant time, several tens of milli-seconds, is required for the trapped ions to lose this Kinetic energy. This means that the scan rate and ion throughput of a LIT-ToF instrument employing this method is severely compromised.
All these prior art methods of trapping ions are applicable to a LIT which is to be used for an ions source of a time of flight analyser. Despite the various trapping options available, the upper scan rate in all cases is limited by the time to cool the ions prior to extraction.
As regards arrangements in which trapping does not occur, poor duty cycle remains a problem. Thus, in a so-called Orthogonal-ToF (O-ToF) arrangement ions are extracted from a field free region external to the ion guide, this is the most common method of introducing ions to ToF analysers. The Orthogonal extraction method was the first method to adapt an ion beam from a continuous ion source into a pulsed ion beam necessary for a time of flight analyser: sections of the beam are pulsed in a direction orthogonal to the continuous beam. This method is commonly known as an “orthogonal Time of flight mass spectrometer” (O-ToF) and it is based on the original work of Wiley & McLaren in 1955 (“Time-of-Flight Mass Spectrometer with Improved Resolution”, Rev. Sci. Instrum. 26, 1150-1157 (1955)). There have been a number of methods for focusing ions into the pulsing region to improve resolving power, for example Boyle et al., in C. M. Anal. Chem. 1992, 64, 2084. The duty cycle is much lower than in the Trap-ToF methods discussed above due to the duty cycle at which the continuous beam may be converted to the pulsed beam. Additionally, a proportion of ions are lost by deliberate cutting/reduction of the ion beam to achieve a desired initial velocity and spatial distribution. Using such methods Orthogonal ToF systems have in recent years achieved mass resolving power of 35 to 40 k. O-ToF is usually coupled to a reflectron. A further disadvantage of Orthogonal-ToF systems is the limitation imposed by the flight time of ions from the ion guide region to pulsing region. The low duty cycle, results in a reduction in sensitivity of the mass analyser.
There have recently been a number of attempts to address the problem of the poor duty cycle of the O-ToF, see for example GB2391697 and CA 2349416 (A1), however the efficiency is still not as high as can be achieved by Trap-ToF methods discussed above.